J. Nannoplankton Res. 27 (2) , 2005 , pp.141-147 © 2005 International Nannoplankton Association
ISSN 1210-8049 Printed by Cambridge University Press , UK141

Impact of a Coccolithus braarudii bloom on the carbonate
system of Portuguese coastal waters
L. Cabec;adas
Instituto do Ambiente, lA, R. da Murgueira 9/9A- Zambujal , Ap.7585- Alfragide, 2721-865 Amadora, Porrugal; leonor.cabecadas@iambiente.pt

A.P. Oliveira
lnstituto Nacional de Investigac;ao das Pescas e do Mar, INIAP-1PIMAR, Avenida de Brasflia, 1449-006 Lisboa, Portugal
Manuscript received 23rd March, 2004; revised manuscript accepted lOth June, 2005

Abstract In the adjacent Tagus and Sado coastal waters (off SW Portugal) , a phytoplankton bloom dominatedby diatoms (up to 1000 cells ml-1) was observed, in spring 2002, under upwelling conditions . Overlapping with thephytoplankton bloom, but spatially differentiated from the diatoms and other phytoplankton groups, a pronounced
development of the coccolithophore Coccolithus braarudii (up to 60 cells ml-1) occurred, associated with a thermally stratified water-mass localised in the Tagus Bay. Some cells of the C. braarudii hyalinus-phase were also
observed. A production of about five tons of calcite was calculated for the upper 30m of the water-column, based on
the integration over depth of a total of 6.7g CaC0 3 m· 2, covering 0.7km2 of the Tagus Bay. An average CaC0 3 precipitation rate of 11.2mmol CaC0 3 ni-2 d- 1 was estimated , resulting in the release of 7 .4mmol C0 2 m-2 d-1 during
the C. braarudii bloom. These results indicate that the studied coastal system contributed an additional source ofC0 2 to the water, and eventually to the atmosphere.

Keywords Coccolithus braarudii , calcification , alkalinity, temperature, coastal waters , upwelling
direction of C0 2 (Robertson et al., 1994).
1. Introduction
Coccolithophores are one of the major phytoplankton
The purpose of this paper is to discuss a Coccolithus
groups in the oceans. They potentially play a unique role braarudii bloom episode associated with upwelling conin the global climate system, mainly through three mech- ditions , and its potential effect on the carbonate system,
anisms: (1) by affecting the carbonate system of sea- namely on CaC0 3 production and alkalinity patterns inwater, involving the production of both particulate inor- the water-column. In addition, quantification of biocalciganic carbon, such as calcium carbonate (CaC0 3) through fication is done in the C0 source/sink context.
2
calcification , and particulate organic carbon(POC) through photosynthesis (Armstrong
38.8"
et al., 2002; Zondervan et al., 2002; Rost &N
Riebesell, 2004); (2) by altering the heatexchange between sea-water and the atmos- 38.7"
phere, due to increased light scattering by
detached coccoliths (Buitenhuis et al.,1996); and (3) by contributing to cloud albe- 38.6'
do, with emissions of dimethylsulphide(DMS) (Buitenhuis et al., 1996). The precip- 38.5'
itation-dissolution of CaC0 3 by calcifying
organisms has an impact on oceanic cycles,in terms of both dissolved inorganic carbon 38.~·
2,1
.IOQn
24
7tDo!
(DIC) and alkalinity (TA), and plays, thus, a
JOOOm
z.o
50irl
J.IQ>n_,significant role in the buffering capacity of 38.3'..... ·roootn 17
J501rl SttaN/
25
28 =
30
3,1 sea-water and its functioning as a sink or arooomsource of carbon dioxide (C0 2 ) to the atmos·9.5'
·9.4'
·9.3'
·9.2'
·9.1'
·9.0'
·8.9'
·8.8"W
phere (Zondervan et al., 2001; Rost & Figure 1: Map of the tudy ite bowing ampling tat ion . A= Transect A (Stations I, 2,
Riebesell, 2004) . Besides the fixation of dis- 3, 4); B =Transect B (Station I, 5, 6, 7)
solved C0 2 leading to a decrease of total
C02 and inorganic nutrients in the water-column, coccol- 1.1 Study area
ithophore production of CaC03 leads to an increase of The present investigation was carried out along thesurface C0 2 pressure (pC0 2) and induces a parallel drop Portuguese coast between 38.25• and 38.7YN and 8.85"
in TA which, in turn, shifts the DIC equilibrium in the and 9 .6o·w, mainly covering the continental shelf off-

.

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.

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Ctn)Vn.

142

Cabet;adas, Oliveira

shore Tagus and Sado Estuaries (Figure 1) . The coastlineof this region is interrupted by large capes (Raso andEspichel) and by pronounced embayments (Tagus andSado). The hydrogeomorphological features are deeply
marked by the intense discharge of the Tagus River, withan annual mean flow of 350m3 s-1, while the Sado's annual mean flow is only 5m3 s- I. The bottom topography of
the area is dominated by the deep submarine canyons of
Lisbon and Setubal, which almost reach the coastline,
thus creating strong bathymetric discontinuities .
Seasonally, wind-driven upwelling events occur in thearea (Fiuza, 1984). South of Lisbon, the upwelled water
exhibits thermohaline (Fitiza & Halpern, 1982) and nutrient characteristics (Brogueira et al., 1994), corresponding
to the North Atlantic Central Water, whereas the thermohaline properties within the Setubal canyon region indicate the presence of Mediterranean water (Ambar, 1983).

1.2 Previous work on phytoplankton
In past years , a number of studies have been undertaken
on phytoplankton temporal and spatial variability along
the Portuguese coast, and specifically on the coastal
waters adjacent to the Tagus and Sado Estuaries (Abrantes& Moita, 1999; Cabe~adas et al. , 1999, 2000; Moita,
2001). In this paper, we focus on the phytoplankton groupof coccolithophores, considering its potential importanceon the biogeochemical cycling of carbon and the lack of
information concerning their ecology in these waters.
For the Portuguese shelf, there are frequent records ofthe coccolithophore species Coccolithus pelagicus (up to
4 cells ml-1) associated with the Iberian upwelling season
and with temperature ranges between 12 and 1TC(Cachao & Moita, 2000). Recently, the larger cell-sized
species from temperate waters have been identified as adifferent species named Coccolithus braarudii (Geisen etal., 2004 and references therein) , therefore this denomination will be adopted throughout this work.

rosette system (SeaBird), equipped with 12 Niskin bottles(81 each) for determination of nutrients (N0 3- and N0 2-),NH4+, P0 4 3- (referred to herein as N0 3 , NH4 and P0 4 ,respectively) and Si(OH) 4 , dissolved oxygen (DO) ,chlorophyll a (Chla) and TA. Nutrient samples were filtered with MSI Acetate Plus filters (0.45Jtm) and analyses
were carried out on a Traacs Autoanalyser, following
Treguer & Le Corre (1975). DO was analysed onboard,
following the Winkler method (Carrit & Carpenter, 1966),
using a whole-bottle manual titration. The coefficient ofvariation associated with the method was 0.8-0 .25%.
Samples for Chla and phytoplankton were collectedfrom surface to bottom at shallow stations and from surface to 50m depth at deeper stations. Ch!a was measured
by filtering triplicate aliquots of 50-lOOm! water-samples
through Whatman GF/F filters. The filters were frozen
immediately and later extracted in 90% acetone for analysis in a Perkin Elmer Fluorometer, using the modified protocol by Lorenzen (1967). Commercial solutions (Sigma
Chemical Company) of Chla were used to calibrate thefluorometer.
TA was determined by potentiometric titration,according to Dickson & Goyet (1994). The 50ml GF/F
(24 hours at 450°C) filtered samples were titrated withHCl (-0.25M HCl solution prepared with lM
Mallinckhodt standard solution in 0.45M NaCI) past theend-point of 4 .5. TA measurements with an accuracy of±4ftmol kg-1 were standardised against reference materials (CRMs) provided by Dr. Dickson of the Scripps
Institution of Oceanography. pC0 2 was estimated frompH and TA data, and calculations were performed with aprogram developed by APO (unpublished data).
Constants were taken from Roy et al. (1993) and Weiss(1974) . The error on the pC0 2 calculation was estimatedto be less than 6jtatm.

2.2 Phytoplankton samples
2. Material and methods
The present investigation was carried out on board the RV
Mestre Costeiro from 29th May to 7th June, 2002 (spring
season) in the study area , where a total of 32 stations were
sampled.
2.1 Physical, chemical and biologicalparameters
Temperature profiles were taken with a SeaBird-CTD
probe calibrated with a high-accuracy reversed thermometer and an AutoSal salinometer. The Mixed Layer
Depth (MLD) corresponded to the depth of water where
the temperature between surface and depth was :2:1 oC.
pH was measured immediately after collecting thesamples using a Metrohm 704 pH-meter and a combination electrode (Metrohm), standardised against NBS
buffers pH6.865 and pH9.180. Precision of the pH-measurements was ±0 .0 1.
Discrete water-samples were taken using a CTD-

Phytoplankton samples were preserved with alkaline
Lugol's solution and stored in the dark at 4'C .
Phytoplankton cells were identified and counted (including large coccolithophore coccospheres) under an inverted Zeiss IM35 microscope , equipped with phase-contrast,by the Utermohl technique (Hasle, 1988).
A 160x magnification was used to enumerate larger
phytoplankton cells in the entire bottom-chamber, whilesmaller cells were counted in two transects with a 400x
magnification. Single coccoliths were disregarded.

2.3 Coccolithophores
A 50ml sample was filtered on a 25mm-diameter
Millipore filter (nominal pore-size of 0.45Jtm) immediately after arrival, using a 25mm Millipore filtration apparatus and under low vacuum. Filters were placed on a
microscope slide and covered with a drop of immersion
oil followed by a coverslip. The clarified filter was examined with a Zeiss Standard Axioskop Mod-2 microscope,Impact of a Coccolithus braarudii bloomusing an oil-immersion lOOx objective, 2.5x optovar lensand lOx oculars. The material observed consisted mostlyof Coccolithus braarudii coccospheres, some cells of its
alternate motile Crystallolithus stage, and small coccospheres not identified (<5Jlm diameter) . In the present
study, 'cell density' numbers, as well as the CaC0 3 estimations , concern C. braarudii calcified cells.
Smaller coccolithophore species (<5Jlm diameter)
were disregarded, taking into account their relatively low
contribution in terms of mass (2.3pg at mean length)
when compared to the much greater per specimen mass
(248pg at mean length) of C. braarudii species.

2.4 Coccolithus braarudii volume,
calcite mass and organic carbon
biomass
Coccolithus braarudii coccosphere size was measured bysemiautomatic image analysis (Zeiss .KS 100 3.0 software) . The measured average coccosphere diameter(dJlm) was 22.5]lm for the sampled population, and mean
coccosphere volume (V = d3Jt!6) was estimated as
6157jlm3. The calcified volume of 3079]lm3, corresponding to -50% of total volume, was converted to mass bymultiplying it by calcite density. An estimation of 8.3ng
CaC0 3 mass per coccosphere was obtained using theYoung & Ziveri (2000) formula.
C. braarudii (coccosphere) was converted to biomass
by using a conversion factor of 233pg C per cell, based onliterature values for Atlantic samples, considering themeasured average cell-size (Holligan et al., 1993) .

2.5 Particulate organic and inorganiccarbon integrations along Transect B
The carbon biomass of total phytoplankton (including
coccolithophores) was estimated from Chla concentrations based on a carbon:chlorophyll ratio of 76, calculated from the linear regression of POC vs Chla (measured
values). Amounts of total phytoplankton carbon biomass
were estimated from the carbon:chlorophyll ratio.
Coccolithus braarudii organic and inorganic carbon were
calculated, respectively, from abundance counts and the
published carbon-per-cell conversion factor, and fromcounts to mass conversion. Total phytoplankton carbon,
C. braarudii organic and inorganic carbon estimations
were submitted to trapezoidal integration from surface tobottom, or 50m depth, at each station of Transect B andexpressed as mg C m-2. These estimations were extrapolated to the whole transect , considering its length(22.8krn) and mean upper-column depth (3lm). Total
phytoplankton carbon and C. braarudii carbon, concerning the studied transect, are expressed in tons .

3. Results
3.1 Oceanographic and chemical
conditions
The present investigation took place in a period of mod-143
Temperature

38.7°

(OC)

N

38.6°

38.5°

38.4°

.

38.3°
-9.5°

-9.3°

-9.1°

<:)

~-

w

-8.9°

Figure 2: Surface temperature CC) distribution covering continental
shelf, offshore Tagus and Sado Estuaries, late spring, 2002

erate but persistent upwelling conditions, revealed bymeans of the Bakun Index (350m3 s-1 krn-l) and by satellite images (Coastwatch, 2003). At surface, temperature
distribution shows clearly the colder upwelled wateralong Transect A (Lisbon- Cape Raso), and a marked
resurgence offshore Sado Estuary reveals temperatures
below 15 ' C (Figure 2). The upwelled water induced sharp
surface temperature gradients ( 14- 18 'C) .
Transect A talion

4
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3

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Distance (km)Figure 3: (a) Vertical temperature CC) distribution and (b) verticalnitrate (}tM) distribution along Tran sect A; (c) vertical temperature CC)
distribution (emphasised line represents Mixed Layer Depth , based on
temperature) and (d) vertical nitrate (JlM) distribution along Transect B

Cabc~adas, Oliveira

144

Along the studied Transects A (Stations 1, 2, 3, 4) andB (Stations 1, 5, 6, 7) , vertical profiles of water temperature and nitrate (shown in Figure 3) evidenced also the
upwelled cold water-mass enriched in nutrients from
depth . Vertical patterns of phosphate and silicate weresimilar to the one of nitrate (not shown).
Along Transect A, the water-column was in general
mixed, while along Transect B, owing to thermal stratification, MLD increased from inshore to offshore, reachinga depth of 28m at the most offshore station (Station 7).
Nearly depleted nutrient conditions were noticed offshorein surface-waters and oxygen supersaturation was always
present with values up to 115 %. By that time, river
plumes were very weak, indicating low discharge fromboth rivers: 46m3 s-1 from the Tagus and 1.0m3 s-I fromthe Sado .

3.2 Phytoplankton abundance
The phytoplankton bloom was dominated by chain-forming diatoms , such as Guinardia striata, Detonula pumila ,Dactyliosolenfragilissimus and Pseudonitzschia spp. The
highest abundance was observed at the surface in thevicinity of the Lisbon submarine canyon head , reaching
concentrations of up to 1000 cells m]- I, which represents
74% of the total phytoplankton (Figure 4a). In this region,
nutrients were almost exhausted (0 .1 <N0 3 <0 .2JLM;
0 .5<NH4 <l.1jtM; 0 .06<P0 4 <0 .llJLM; 0.4<Si(OH) 4<l.4JLM) and the water-column was stratified. MLD varied between 10 and 20m.
Spatially differentiated from the diatoms occurred a
proliferation of the coccolithophore Coccolithus

braarudii. At the surface, offshore Tagus Estuary, C.
braarudii densities of 22.5 cells rnl-1 were attained, whilesouth of that area, close to Cape Espichel, densities of 1
cell ml- 1 (Figure 4b) were registered . The nutrient regimen for the first area was influenced by the Tagus Estuary
plume (4.0<N0 3<5.2JLM; 0.4<NH4<1.0JLM; 0.6<P0 4 <0.8JLM; 1.6<Si(OH)4 <2.3JLM), while the second area
showed concentrations typical of an offshore region(0.1<N0 3<0.5ytM;
0.04<P0 4 <0 .2<NH 4 <0.3JLM ;0 .09JLM; 0.3<Si(OH) 4 <0.4ytM). Across both areas, MLD
varied between 5 and lOm .
C. braarudii highest densities occurred along Transect
B, showing densities of up to 60 cells rn1-1 at the most offshore station (St.7) between 30 and SOm depth (Figure Sa)
where nutrient concentrations were quite low (O.l<N0 3<
2.3JLM; O.l<NH 4 <0 .2JLM; 0.09<P0 4 <0.20JLM; 1.0<
Si(OH) 4 <1.3JLM) and the MDL deeper (27m). In contrast,
total phytoplankton reached higher abundances inshore at
subsurface (up to 240 cells ml-1), showing an opposite
spatial pattern to that observed for the coccolithophores(Figure Sb) .
Tran eel 8 tations

7
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6

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phytoplankton(cells ml -1 }

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5Distance (km)

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Figure 5: Transect B distributions. (a) Vertical Coccolithus braarudii(cell s ml"' ) ; (b) vertical total phytoplankton (cells ml-1)

38.4•
< 100

38.

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~·O '-;

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38.3°
(b)

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-9.3"

-9.1°

-8.9" w

Figure 4: Surface distributions in Tagus and Sado adjacent coastal
waters , late spring , 2002. (a) Total phytoplankton (cells mJ- 1); (b)Coccolithus braarudii (cells mi-1)

3.3 Phytoplankton organic and inorganiccarbon estimations for Transect B
For the entire Transect B, an amount of ~s tons of POC
was estimated for the total phytoplankton biomass(including
coccolithophores).
Consideringthat
Coccolithus braarudii corresponded to 0.1 ton of organiccarbon, it can be roughly inferred that this species
accounted for 2% of the total phytoplankton biomass.
Integration of C. braarudii CaC0 3 over depth yieldeda total of 6.7g CaC0 3 m- 2 for the area defined by Transect
B . Considering the referred area, we estimate ~s tons ofcalcite were produced in the upper 30m of water.
3.4 The carbonate system
A striking feature in the alkalinity distribution of the stud-Impact of a Coccolitlms braarudii bloom

ied waters was the obvious difference in pattern between
the areas north and south of Cape Espichel. North Cape
Espichel and adjacent to the Tagus Estuary, TA displayed
values as high as 4600Jtmol kg-I, while south of that cape,and adjacent to the Sado Estuary, TA showed values ranging from 2300 to 2600J1mol kg-1 (Figure 6a). The two
estuarine sources have exhibited quite different alkalinities (Tagus: 3560J1mol kg- I; Sado: 2620Jtmol kg-l),revealing differences in the transported material to thecoastal waters. From TA and pH values, DIC concentrations were determined by using Roy et al.'s (1993) set of
constants. A similar pattern of distribution to that obtained
for TA is observed, with values ranging from 2050 to4200J1mOl kg-1. Both TA and DIC behaved non-conservatively. The entire study area exhibited elevated values ofTA and DIC, which were probably related to these
upwelled waters enriched in TA and DIC. South of Cape
Espichel and adjacent to the Sado Estuary, values werelower, since the upwelled water indicates the presence ofMediterranean Water while north of that cape and adjacent to the Tagus Estuary, the upwelled water corresponded to the North Atlantic Central Water.
TA

lt-m<>l"-11' 11

peo,(lllltm)

38.6°

38.5•

•§

38.4

38.3
(b)

-s.s•

-9.3°

-8.9" w

Figure 6: Distributions in Tagus and Sado adjacent coastal waters, latespring, 2002. (a) Surface TA (jtmol kg-1); (b) pC0 2 (jtatm)

It is clear from Figure 6b that the entire study area wassupersaturated with respect to pC0 2 (470- 1770Jlatm) ,with oversaturation with respect to atmospheric equilibrium (presently ~360}1atm) close to 490%. Oversaturationof pC02 was probably due to the transport of higher quantities of inorganic carbon from deep, colder upwelledwater. This feature was more striking along Transect A,
although induced remineralisation due to the presence ofa peak in organic matter (not shown) may also have

145

accounted for the elevated pC0 2 values (up to 1770Jlatm).

4. Discussion
The described scenario seems representative of a declining bloom episode, taking place during late spring incoastal waters under upwelling conditions , showing nutrients, to a certain extent, exhausted throughout the watercolumn . However, satellite images clearly point out thatupwelling conditions persisted during the phytoplankton
bloom until the end of June. Diatoms proliferated wellthroughout turbulent waters , while coccolithophore
development required more stable conditions and coincided with lower nutrient levels . Apparently, coccolithophores found optimal growth conditions close to afrontal zone established between a deep, colder, upwelled
mixed water-mass, located alongshore Lisbon-Cape
Raso, and a thermally stratified water-mass which occupies the Tagus and Sado Bays (Figures 4a, b) . Some cellsof Crystallolithus braarudii (Coccolithus braarudii alternate motile stage), clearly exhibiting flagella and crystalloliths , were observed at Transect A, associated with acolder and mixed , upwelled water-mass (Figure 2), probably indicating that the motile phase could have been an
important component of the Tagus Bay population. The
simultaneous occurrence of C. braarudii and its alternate
motile Crystallolithus stage have never been reported for
the Portuguese shelf. The growth of these motile cells
could have been trigged and amplified by the local environmental conditions over the non-motile C. braarudii
population and are reflected , to a certain extent, in thealkalinity distribution in the area (Figure 6a) . As a matterof fact , TA distribution at Transect A (especially Stations
2, 3 and 4) and B (particularly Stations 6 and 7) cannot be
explained only by a variable alkaliruty estuarine source,
alkalinity changes due to biological activity, and upwelledwaters with high TA. Considering that a mean rain ratio(the ratio of particulate inorganic to organic carbon inexported biogenic matter) of~ 1 was achieved in Transect
B waters, revealing that photosynthesis and calcification
processes were in equilibrium, the sharp alkalinity rise insurface-waters adjacent to the Tagus Estuary could possibly be related to the dissolution of C. braarudii cells. At
this transect, the ~5 tons of calcite produced in the upper
30m of water confirm that significant CaC0 3 deposition
can occur in coastal temperate ecosystems. Holligan et al.
(1993) estimated 7.2xl0 4 tons of calcite in the upper 60m
of water for a bloom of coccolithophores (40g CaC03m·2) covering 7200km2, which he mentions to be an orderof magnitude smaller than the annual CaC0 3 production
for the European shelf by coccolithophores. Consideringa sinking rate for C. braarudii coccospheres of about 7m
d-1 (Young , 1994), an average CaC03 precipitation rate ofl.lg CaC03 m-2 d-1 can be estimated along the referredtransect, this being a value within the ranges of carbonate
production found in the literature for coccolithophores(Fernandez et al ., 1993; Holligan et al., 1993) . Actually, a146

Cabe~adas, Olheira

calcification rate of 1.7g CaC0 3 m- 2 d-1was observed during a coccolithophore bloom survey in the NE Atlantic(Holligan et al., 1993).
Concerning Transect B, and using the molar ratio (Y)of C0 2 released versus CaC0 3 precipitated(Frankignoulle et al., 1994), which attained the meanvalue of 0.66, it was possible to calculate the fraction of
C02 released to the water during calcification. Throughthe 11.2mmol CaC0 3 m-2 d-1estimated for the total population of C. braarudii, a value of7.4mmol C0 2 m- 2d-1was
calculated, indicating that the calcification process constitutes an additional source of C02 to the water and, eventually, to the atmosphere.
C. braarudii calcification/dissolution in the surface
layer modifies upper-ocean alkalinity and directly affects
air/sea C02 exchanges. The results obtained are in agreement with recent works (e.g. Rost & Riebesell, 2004),
which indicate that productivity and di stribution of coccolithophores are sensitive to C02-related changes inenvironmental conditions, both directly through acidification of surface sea-water, and indirectly through increasing upper-ocean thermal stratification .

Acknowledgements
We acknowledge the captain and the crew of the RV Mestre
Costeiro for their excellent support and co-operation. We are
grateful to L. Palma Oliveira , A .M. Correia and A. Pereira for
their technical and sampling assistance. Thanks are due to M.
Nogueira for the CTD data. We are indebted to the Coastwatch
services (namely in the person of Dr. Odile Fan ton d' Andon and
Grigor Obolenski, from ACRI-ST) for the supply of the satellite
images . The authors thank !NAG for providing hydrometricdata. We also wish to acknowledge the reviewers, M. Cachao
and M. Geisen, and the Editor J. Lees. This study was funded by
the European Commission, Programa Operacional do Ambiente,
POA Project 1.3.000 12.

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